enable_hypertable_compression/2 :segment_by option needs to support a list of columns

[davydog187]

Currently we are only supporting a single column by string/atom, e.g.

enable_hypertable_compression(:my_table, segment_by: :column_a)

But we should be allowing a list of columns

enable_hypertable_compression(:my_table, segment_by: [:column_a, :column_b])